Traditionally, all of the various components of an integrated circuit were powered at a single voltage level. However, in more recent technologies, different components of an integrated circuit are powered from different voltage sources. For example, in some new technologies the core of an integrated circuit, such as the memory or logic components, is powered off of one voltage source, and the input output components of the integrated circuit are powered off of a different voltage source. Typically, the core voltage level, generally designated as VDDcore, is nominally 1.2 volts or less and the input output voltage level, generally designated VDDio, is nominally 3.3 volts. An integrated circuit that is powered by more than one voltage source typically uses a level translator, also called a voltage level shifter, to step digital signals between the core voltage level and the input output voltage level.
However, as integrated circuit designs have evolved, the demands on level shifters have also evolved. For example, as the core voltage has dropped in potential, it has approached the lower-most levels at which the relatively voltage tolerant input output transistors function, also known as the threshold voltage of the input output transistors. In other words, the signals received from a voltage sensitive core transistor by a voltage tolerant input output transistor may be at so low a level, that the voltage tolerant input output transistor may not operate properly or reliably in response to the lower voltage signal. Thus, there is a need for an alternate signal to trigger the operation of the relatively voltage tolerant input output transistors in level shifters.
Conversely, as the core voltage has dropped in potential, the relatively voltage sensitive core transistors have become increasingly sensitive to the relatively higher input output voltage potentials. In other words, the signals received from a voltage tolerant input output transistor by a voltage sensitive core transistor are at so high a level, that the voltage sensitive core transistor may be damaged in response to the higher voltage signal. Thus, there is a need for a reliable signal threshold to limit the signal to the relatively voltage sensitive core transistors in level shifters.
There is a need, therefore, for new designs of level shifters.